There is an increasing use of global distribution systems such as the Internet for distribution of digital assets including music, film, computer programs, photographs, games and other content. There is also a concurrent increase in the unauthorized copying, or pirating, of digital content causing considerable economic losses to content providers. Effective countermeasures are important to the viability of businesses engaged in the distribution of digital media.
Many types of countermeasures have been developed to prevent or deter the creation and distribution of unauthorized copies of digital media. Some countermeasures, such as encryption, are directed at preventing pirating in the first place, while others try to locate the source of the unauthorized copies. For example, technologies such as digital watermarking and traitor tracing are directed to finding the sources of unauthorized copies. Digital watermarking involves the addition of a unique mark on each copy of distributed digital content. If an unauthorized copy of the watermarked content is found the watermark may be used to identify the pirate.
In general, a digital watermark may be a pseudo-noise pattern representing the identity of the user, which is superimposed on the content to be distributed. Later, when an unauthorized copy is found, the presence of a particular watermark pattern should reveal the identity of the traitor who has compromised the content.
In addition to using watermarks to determine the source of pirated content, watermarks can also be used to prevent the use of unauthorized content. For example, current watermarking technologies, such as the DVD audio format, and the new blue-laser HD video DVD format, use audio watermarks for copy control purposes. A studio may put an inaudible watermark in the soundtrack of the movie when they release the movie as an authorized DVD. A player, if it detects the watermark in the soundtrack of an unauthorized format (for example, an in-the-clear movie masquerading as a home video DVD), it stops playing.
This technique is very valuable to reduce commercial piracy of movies. However, there is the possibility of a potential consumer problem, illustrated by the following example, referred to as the “birthday party problem”, or simply “birthday problem”. Assume that a consumer is making a home video of a child's birthday party. It happens that the children are also watching a movie on the TV, while they are being filmed. The soundtrack from the movie will be inadvertently recorded along with the normal audio from the party. This movie soundtrack may have an audio watermark embedded into it. Later, when the movie of the birthday party is played, the player may detect the watermark from the movie soundtrack and, thinking it is a pirate disc, stop playback of the home video.
Heretofore, one technique used to mitigate this problem is to reduce the sensitivity of the watermark detector, in terms of the length of the period of detection that must be observed before action is taken. For example, the player might be required to detect the signal continuously for ten minutes before stopping play. This is not a totally satisfactory solution, however, because innocent home videos could still hit the ten minute limit. Furthermore, pirates may work to reduce the watermark signal in their pirate videos, and might be able to make the watermark signal intermittent enough to avoid the relaxed detection criteria.
Accordingly, there is a need for systems and methods for solving the “birthday” problem with watermarked content. There is also a need for such systems and methods which can solve the birthday problem, are effective in virtually every case, do not make it easier for pirates to circumvent the watermarks, and have a low probability of error.